Which of the following nutrient forms is the most immobile in soil?

Phosphate is considered the most immobile nutrient form in soil due to its strong tendency to bind to soil particles, particularly in the presence of aluminum and iron oxides, which are common in many soils. This binding reduces its availability to plants because it becomes less soluble and is less able to move through the soil profile.

In contrast, other nutrients like nitrate and sulfate are more mobile in the soil because they are negatively charged and can readily leach with water movement. Calcium, being a cation, can move within the soil but not to the extent that phosphate does in terms of binding and becoming less available. Thus, phosphate's strong affinity for soil particles makes it the least mobile among these nutrient forms.